1) Field of the Invention
The present invention relates to a structure for fastening by a screw, and more particularly to a structure for fastening thin plates made of, for example, metal, by the screw.
2) Description of the Related Art
FIG. 8 is a sectional view of a prepared hole (burring hole) formed by a conventional protruding method. In the case of a first material 24 having a thickness of 1.2 millimeter to 0.8 millimeter, a protruding height of 25 millimeter is secured by protruding the first material 24 using the conventional protruding method.
In a process of a rolled threading, if an ordinary tapping screw species 3 of a forming type, which is prescribed by JIS (Japanese Industrial Standard) is used, a tip 26 of the prepared hole widens as illustrated by dotted lines 27 in FIG. 8. Thus, it is impossible to secure a useful thread length, and results in a decrease in a failure torque of a female thread. Furthermore, a fastening performance is largely influenced by variations of a shear drop portion 28 in a screwing position, which is caused by the protruding method. Moreover, a great pressing power is required to perform a burring.
However, it has been believed impossible to apply a method of fastening by a screw to a thin metal sheet that is as thin as, for example, 0.6 millimeter, even if the burring is applied to the first material 24, if the tapping screw species 3 prescribed by JIS is used. In other words, it has been believed that fastening by a screw is impractical as the failure torque of the female thread is too low. The failure of the female thread of the burring type that uses the metal sheet having a thickness of 0.8 millimeter occurred in less than ten repeated fastenings, when the screw species 3 is fastened.
There is a method of fastening the thin metal sheets with a screw species 1 without burring. However, it is impossible to fasten repeatedly because an axial force is too low. The axial force is a force to press a second material to the first material. In other words, it is impossible to apply the method to a portion where securing of the axial force (a contact force) and prevention of loosening are required, from a viewpoint of safety rules, which are applied to all positions to be screwed in office automation apparatuses.
Meanwhile, in view of an environmental consideration, an increasing reference number of apparatuses has been recycled. Repeated usability of the screws is required for recycling. Moreover, energy saving effect is expected by reducing a weight of the apparatus.
A tapping screw of the forming type for the thin metal sheets that has a single-start thread at a lead portion and double-start threads at the useful thread portion is conceived as a screw that can fasten the thin metal sheets without the burring.
The tapping screw with the single-start thread and the double-start threads, however, is flawed for being low in axial force. The failure of the female thread can occur in an attempt to increase the fastening torque under a condition that the thread is relatively larger in diameter, and especially the thickness of the sheet metal is as thin as 0.6 millimeter.